Equity Offerings - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended			9 Months Ended
	Jul. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017
Class Of Stock [Line Items]
Common stock issued		5.8	2.1
Common stock average price per share	$ 42.50	$ 59.75	$ 17.84
Net proceeds from issue of common stock		$ 327.4	$ 37.3	$ 354.0
Underwriting discounts, commission and offering expenses		$ 17.6	$ 0.2	$ 20.0
Underwriting discount and commission per share			$ 1.60
Underwritten Public Offering [Member]
Class Of Stock [Line Items]
Common stock issued	8.8
Underwritten Offering [Member]
Class Of Stock [Line Items]
Common stock issued	1.2